After a wellbore has been formed into the ground and the casing has been cemented into place, the hydrocarbon containing zone usually is communicated with the casing interior by forming a plurality of perforations through the casing which extend radially away from the casing and out into the formation, thereby communicating the hydrocarbon producing zone with the interior of the casing.
It is common practice to run a jet perforating gun downhole and to fire the gun by the employment of a gun firing head which is actuated by a bar dropped down through the interior of the tubing string. Completion techniques involving this known completion process are set forth in U.S. Pat. Nos. 3,706,344 and 4,009,757.
A bar actuated firing head cannot be used in certain situations and sometimes it is desirable to be able to detonate the charges of a perforating gun without the use of a bar. Particularly it would be advantageous to actuate the gun by effecting a pressure within the pipe string or annulus or both, but a gun firing head which could be detonated in response to pressure effected within the borehole has been considered to be highly dangerous by many logging and completion engineers for the reason that leakage across some of the critical seals of the firing head could inadvertently detonate the firing head and prematurely explode the shaped charges of the gun. Should this misfire occur at an inappropriate time, untold damage could be done to the wellbore if, for example, the explosion occurred while running the gun into the hole, or if the explosion occurred before proper flow passsageways back to the surface had been provided for the completed formation. If a pressure actuated gun is to be safe, it is necessary that the firing head be unable to detonate the shaped charges until the gun has been lowered downhold and properly located relative to the formation to be completed.
U.S. Pat. No. 3,189,094 to Hyde discloses a hydraulically operated firing apparatus on a gun perforator for purposes of formation testing. The firing apparatus assembly includes a tubing string having a conventional formation tester valve in a housing and a conventional packer secured below the housing. Firing apparatus housings, along with the gun perforator, are series connected to the tubing string below the packer. In conducting a formation test, the assembly is lowered into a fluid filled wellbore so that, externally, all parts of the assembly are subjected to the submergence pressure exerted by the fluid in the well. The formations tester valve is initially closed so that the pressure within the empty tubing string is essentially at atmospheric pressure. When the packer is set, the zone opposite the gun is isolated from the region above the packer. Thereafter, when the formation tester valve is opened, the zone opposite the gun is exposed essentially to atmospheric pressure, or at least to a pressure which is greatly lower than the submergence pressure of the fluid in the well. Although various embodiments of the firing apparatus are disclosed, all of the embodiments utilize the submergence pressure to arm the firing apparatus during descent of the assembly and then utilize the low pressure condition created when the packer has been set and the formation tester valve opens to cause a pressure differential which operates the firing apparatus and fires the gun. The gun perforator penetrates the surrounding formation so that the formation fluids flow into the tubing string to complete the formation testing operation.
The present invention overcomes the deficiencies of the prior art.